Combined Solar/Thermal (CHP) Heat and Power for Residential and Industrial
Buildings
Priority
[0001] This application is a continuation-in-part of U.S. Provisional Application
No. 61/198,256, filed November 4, 2008, the contents of which are incorporated
herein by reference.
Field of the Invention
[0002] The present invention relates to solar powered devices and methods of
converting solar power into useful forms of energy.
Background
[0003] There is a long recognized and continuing need for cost effective
renewable energy sources. With this goal, significant efforts have been made to
develop cost effective solar powered generators to harvest solar energy. The main
focus of these efforts have been to make high efficiency low cost solar panels.
[0004] Solar panels are photovoltaic devices designed to convert solar energy
directly to electricity. Basic solar panel technology is based on p-n junctions. The
difference in charge carrier concentration between p- and n- doped regions of a
semiconductor material cause charge carriers to diffuse, thereby creating a static
electric field within the semiconductor. The semiconductor has a band gap energy
which is the energy difference between the minimum of its conduction band
maximum of its valence band. Many semiconductors the band gap energy that lies
within limits of the solar radiation spectrum. Photons with energy greater than the
band gap energy can be absorbed by the semiconductor and raise charge carriers
from its valance band to its conduction band. The excited carriers flow as a result of
the electric field and provide electrical power.
[0005] Solar panels in current use can be broadly divided into crystalline silicon
and thin film technologies. Crystalline silicon is a relatively poor absorber of light and
requires a comparatively large thickness (several hundred microns) of material in
comparison to materials such as Cadmium Telluride (CdTe) and Gallium Arsenide

(GaAs) used in thin film technologies. Presently, crystalline silicon solar panels
provide higher efficiencies than thin film solar panel, but are more expensive to
make. Good conversion efficiencies for solar panels commercially available at this
time are in the range from 14-19%. Higher conversion efficiencies are possible.
[0006] A maximum efficiency for converting un-concentrated solar radiation into
electrical energy using a single junction solar panel at room temperature is about
31 % according to the well known Shockley-Queissar limit. This limit takes into
account a thermodynamically unavoidable rate of carrier recombination and a
mismatch between the band gap energy of the semiconductor and the solar energy
spectrum.
[0007] The mismatch relates to the quantization of energy in light. Wavelengths
of light with energy below the band gap energy cannot excite charge carriers.
Wavelengths of light with energy above the band gap energy can excite carriers, but
the energy in excess of the band gap energy is rapidly converted to heat. Band gap
energies around 1.3eV provide the highest theoretical efficiency for a single-junction
solar panel at room temperature.
[0008] The Shockley-Queissar limit for a single-junction solar panel can be
exceeded by providing multiple junctions. A typical multi-junction solar panel
comprises a layered stack of two or more semiconductor materials having different
band-gap energies. The uppermost layer has the highest band gap energy. Ideally,
the uppermost layer absorbs the portion of the spectrum with energy equal to or
greater than the upper layer's band gap energy while passing longer wavelengths for
use by the layers beneath.
[0009] Optical transparency for the layered structure generally requires that all
layers have similar crystal structure and lattice constants. A lattice constant
describes the spacing of the atom locations in a crystal structure. Mismatch in the
lattice constants between different layers tends to creates dislocations and
significantly deteriorates the efficiency of a multi-junction solar panel.
[0010] While the choice of materials for multi-junction solar panels is constrained,
many suitable combination have been found and shown to outperform single junction
cells. By suitably dividing the absorption spectrum, excellent results have been
obtained with two, three, and four junction cells. For example, a two junction cell
comprising InGaP (1.9 eV) and GaAs (1.4eV) held a record efficiency near 30% in

the 1990's. A three junction cells comprising GalnP (1.85 eV), GaAs-layer (1.42 eV)
and Ge (0.67 eV) has been used to demonstrate efficiencies near 40%.
[0011] Another way to improve solar panel efficiencies is by concentrating
sunlight onto the solar panel surface. Aside from the obvious benefit of providing
more light per unit area, the direct light provided by a concentrator (as compared to
the diffuse light received by a panel directly exposed to the sun) allows for a higher
efficiency. 41 % is the theoretical limit for a single junction cell and 55% for a two
junction cell. For direct sunlight, the optimal bandgap energy at room temperature is
1.1 eV For a two junction cell in the standard series configuration, a 0.77eV, 1.55eV
pairing is the approximate optimum. For three junctions, 0.61 eV, 1.15eV, and 1.82
eV approximates the ideal as reported by M.A. Green in . Third-Generation
Photovoltaics: Advanced Solar Energy Conversion, pp 60-63 (Springer: Heidelberg,
2003).
[0012] A further improvement to enhancing the electrical conversion efficiency
involves deriving electrical energy from the excess energy absorbed when an
electron is excited by a photon with energy in excess of the band gap energy.
Initially, this energy is retained by the carriers, resulting in "hot carriers". There are
two fundamental ways to use the hot carriers for enhancing the efficiency of
electrical energy production. One way produces an enhanced voltage and the other
produces an enhanced current. The former requires that the carriers be extracted
before they cool, while the latter requires that hot carriers having sufficient energy to
produce a second electron-hole pair through impact ionization. For either process to
be effective, it must be carried out at a rate competitive with the rate of carrier
cooling, which is itself very fast.
[0013] The rate of carrier cooling can be greatly reduced by producing the
carriers within a nanocomposite material that alters the relaxation dynamics through
quantum effects. Nanocomposite materials include quantum wells, quantum wires,
and quantum dots. These structures confine the carriers.to regions of space that are
smaller than or comparable to the carrier's deBroglie wavelength or to the Bohr
radius of excitons in the semiconductor bulk. Quantum dots are most effective in this
regard.
[0014] Quantum dots consisting of very small crystals of one semiconductor (e.g.
Indium Gallium Arsenide) within a matrix of another semi-conductor (e.g., Gallium

Arsenide) can slow carrier cooling to the point where impact ionization becomes
significant. Impact ionization when a hot carrier gives up some of its energy to excite
a second carrier from the valence band to the conduction band while itself retains
sufficient energy to remain in the conduction band. Impact ionization can also be
achieved by quantum dots consisting of very small semiconductor crystals dispersed
in an organic semiconductor polymer matrix.
[0015] Hot carrier extraction can be achieved by ordering the quantum dots in
closely spaced three-dimensional array with sufficiently close spacing for strong
electronic coupling and the formation of mini-bands to occur The mini-bands allow
long-range electron transport. The mini-bands provide fast enough transport for the
hot carrier current to be drawn off at a potential above the normal conduction band
potential. To understand this mechanism, it may help to note that the hot carrier
energy spreads among all the carriers in the conduction band on a shorter time scale
than the timescale on which the energy spreads towards thermal equilibrium in other
ways. Thus the entire carrier stream is "hot".
[0016] To avoid confusion with the above mechanisms, it is worth noting there is
another use for quantum wells in enhancing solar panel efficiencies. Quantum wells
can be used to adjust and finely tune the band gap energies of the semi-conductors
into which they are incorporated. This allow semi-conductor band gap energies to
be adapted to better match the solar spectrum and provides flexibility in selecting
materials.
[0017] Still further, nanocrystals within a semiconductor composite have highly
size dependent band gap energies. These can be used to make available to charge
carriers energy states intermediate the valance and conduction bands of the matrix
materials. These intermediate bands allow the composite to achieve electrical
conversion of photons with energy below the band gap energies of the matrix
semiconductors through a two step process of exciting charges from a valance band
to the intermediate bands and from the intermediate bands to a conduction band.
[0018] Many of the foregoing structural enhancements are only economical in
conjunction with solar concentration. Commercially available solar concentrators
provide solar energy with concentrations of 500. While such high concentration
justifies the use of highly engineered semiconductor materials it introduces the
problem of managing intense heat Heating is very detrimental to solar panel

performance.
[0019] The theoretical maximum efficiencies quoted above all diminish with
increasing temperatures. All solar panels undergo diminishing efficiency with
increasing temperature. According to the National Aeronautics and Space
Administration (NASA), as reported in U:S. Pat. No. 7,148,417, a typical silicon solar
panel loses about .45% power per degree centigrade of increasing temperature.
Above 250X, silicon solar panels produce essentially no power. GaAs solar panels
fare somewhat better, losing only about .21% power per degree Celsius. Multi-
junction thin film solar panels generally fared even worse because the layer
thickness are generally carefully matched to equalize currents produced by each
layer. Even a 5% mismatch can severely disrupt the multi-Junction solar panel's
operation. M.A. Green in Third-Generation Photovoltaics: Advanced Solar Energy
Conversion, p, 63 (Springer: Heidelberg, 2003). The routine solution of this problem
is to provide cooling.
[0020] Solar panels have been used to provide hot water for domestic use in
addition to electricity. As noted by U.S. Pat. No. 2004/0055631, using the solar
panel in this manner requires operating the solar panel at a temperature of at least
about 60 °C, which significantly compromises the cell's electrical production
efficiency. The solution proposed by that application is make the solar panel to a
portion of the solar energy spectrum with energy below a semiconductor's band-gap
energy. The solar panel is insulated from the heating elements, which utilize a
portion of the solar energy spectrum that could not be converted to solar panel. The
solution is said to be more space efficient than the alternative of using separate solar
energy collectors for electricity production and water heating. Another way to go
generate hot water is to draw heat from the solar collection system. When high
degrees of solar concentration are used, the waste heat can be considerable.
Summary
[0021] The invention provides solar powered generators and associated methods.
One aspect of the invention is a solar powered generator comprising a solar panel
and a thermoelectric device adjacent to and below the solar panel. The hot junction
of the thermoelectric device is in close thermal coupling with the bottom side of the
solar panel. A heat sink is placed in contact with the cold junction of the

thermoelectric device to cool it. The thermoelectric device has n- and p-type legs
that comprise one or more segments of doped semi-conductor material. At least one
of the segments is formed of a nano-composite material in which quantum
confinement of carriers substantially reduces the segment's thermal conductivity.
[0022] In general, it is not desirable to place a thermoelectric device in between a
solar panel and a heat sink. It is both simpler and more energy-efficient to cool the
solar panel directly. The present invention creates an exception to the general rule.
In the first place, the invention mitigates the energy conversion efficiency loss
introduced by the thermoelectric device by using recently developed materials for the
thermoelectric device that greatly enhance the performance of such devices.
Second, the inventor recognizes that in some situations an adequate heat sink to
maintain the solar panel at a desired temperature cannot be provided in a practical
manner. In those situations, a large degree of heating may be unavoidable. Where
the only available heat sink is inadequate to effectively maintain the temperature of a
solar panel within a narrow operating temperature range, the present invention can
perform better because the thermoelectric device can keep the solar electric
conversion efficiency high even as the solar generator heats up due to
underperformance of the heat sink. The invention is also useful in that the solar
panel can provide cold start performance for what is essentially a solar heat-powered
thermoelectric generator.
[0023] In a method according to the invention, a solar panel is configured to
receive concentrated sunlight and a thermoelectric device is configured to draw heat
from the solar panel and transfer it to a heat sink. At dawn and other times when
there has not been much sunshine, the solar panel produces more power than the
thermoelectric device. On sunny days, the solar panel is allowed to heat to a large
degree. As the solar panel heats, it produces progressively less electricity while the
thermoelectric device produces progressively more electricity. The solar panel is
allowed to heat to a high temperatures. The solar panel can be allowed to reach
temperatures at which the thermoelectric device is the dominant mode of electricity
generation.
[0024] Another aspect of the invention is a solar powered generator comprising a
photovoltaic device and a thermoelectric device of unitary construction. Either the
photovoltaic device comprises layers of semiconductor material grown on

components of the thermoelectric device or the thermoelectric device comprises
layers of semiconductor material grown on components of the photovoltaic device.
This unitary construction reduces material requirements and enables the hot junction
of the thermoelectric device to heat quickly to temperatures at which thermoelectric
energy conversion is efficient.
[0025] A further aspect of the invention also relates to a solar powered generator
comprising a photovoltaic device and the thermoelectric device wherein the
thermoelectric device is adjacent to and below the solar panel with the hot junction in
close thermal coupling with the bottom side of the solar panel. The thermoelectric
device has n- and p-type legs that comprise one or more segments of doped semi-
conductor material. The thermoelectric device is conformal with the surface of the
solar panel and spans approximately the same area. The legs of the thermoelectric
device are thinly dispersed in a matrix of highly thermally insulating material selected
from the group consisting of vacuum, gas, and an aerogel. The legs occupy less
than 10% of the cross-sectional area and the insulation occupies more than 90%.
This configuration greatly reduces the amount of semiconductor material required for
the legs, which is particularly important if nanocomposite materials are used.
Because the legs are evenly dispersed with respect to the solar panel surface, very
small, and closely spaced, and because the legs are very short, the legs effectively
cool the solar panel in spite of their low spatial density and low thermal conductivity.
[0026] The primary purpose of this summary is to present some major concepts
of the present disclosure in a simplified manner that will facilitate understanding of
the more detailed description and claims that follow. This summary cannot be
comprehensive and cover every idea and detail that may be considered inventive or
serve to delineate that which is inventive. Other ideas and details and ways of
generalizing and applying the foregoing concepts will be conveyed by the following
description, the drawings, and this disclosure as a whole. The ultimate statement of
what the inventor is claiming is reserved for the claims that follow. Those claims
may be amended through the ordinary course of patent prosecution.
Brief Description of the Drawings
[0027] The accompanying drawings use reference numbers according to certain
conventions. Identical reference numbers appearing in different figures indicate the

same element is being shown in different positions, uses, or perspectives. Where
the two reference numbers are different but identical in their two least significant
digits, a relationship is still indicated: the referenced objects are related as being of
the same genus or related as species and genus. The drawings themselves and the
context of the description will clarify which relationships apply and whether remarks
about one element are equally applicable to related elements. Trailing letters are
used to distinguish repeated elements within a single drawing or example.
[0028] Figure 1 is a schematic illustration of an exemplary solar powered
generator 100 operating with heat sink and sunlight.
[0029] Figure 2 is a schematic illustration of an exemplary solar panel and
thermoelectric device of integral construction.
[0030] Figure 3 is a schematic that illustrates segmenting in the design of a
thermoelectric device.
[0031] Figure 4 is a plot showing the thermoelectric figures of merit as a function
of temperature for several semi-conductor material
[0032] Figure 5 is a plot showing the thermoelectric figures of merit of an p-Si/p-
SiGe nano-composite.
[0033] Figure 6 provides a finite state machine diagram of the solar powered
generator 100 operating in a preferred manner.
[0034] Figure 7 corresponds to a cross-section through line A-A' of Figure 2, and
illustrates the legs of a thermoelectric device widely, evenly, and finely distributed in
a highly thermally insulating matrix.
[0035] Figure 8 illustrates an exemplary domestic hot water heating and power
generation system employing concepts from each of the foregoing figures.
Detailed Description
[0036] Figure 1 provides a schematic illustration of an exemplary solar powered
generator 100 that produces electricity from sunlight 109.. The solar powered
generator 100 comprises an optional solar concentration system 101, a solar panel
102, and a thermoelectric device 103. The generator 100 requires a heat sink 104 to
operate. The heat sink 104 may be provided as part of the solar powered generator
100. The generator 100 produces power from both the solar panel 102 and the

thermoelectric device 103. These sources are generally brought to the same
voltage, combined, and coupled to a load.
[0037] The solar concentration system 101 can be any suitable device that
functions to concentrate sunlight. The solar concentration system 101 can provide a
low, medium, or large degree of solar concentration. A low degree would be a
concentration factor, f, in the range from about 2 to about 10. A medium degree of
concentration would be in the range from about 10 to about 100. A concentration
factor over 100 would be considered high. In the absence of solar concentration, f is
1.
[0038] Solar radiation is incident upon the earth's surface with a concentration
that peaks at about 1.3 kW/m2. This value is sometimes used as unit of solar energy
flux density "1 sun". A solar concentration system 101 with a concentration factor, f,
illuminates the upper surface 105 of the solar panel 102 with an energy densities
peaking at about f*1.3 kW/m2 (f suns). The actual concentration of sunlight provided
by the solar concentration system 101 at any given time may vary according to
factors such as the position of the sun in the sky, but every solar concentration
system can be expected to have a fairly well-defined maximum concentration factor
representing its capability and a maximum intensity of sunlight it can be expected to
deliver.
[0039] Solar concentration can be achieved with arrangements of reflective
mirrors and/or refractive lenses. The preference here is for the simplest systems
that still provide high solar concentration. Spectrum splitting is not required. The
solar concentration system 101 may comprise a solar tracker to adjust positioning to
keep focus as the sun moves. The solar concentration system 101 may have a
cooling system to keep its components from heating excessively under intense light.
The system 101 may comprise a bundle of optical fibers, whereby the solar
concentration system 101 can be positioned remotely from the other components of
the solar powered generator 100. In this regard, it should be appreciated that
references to a "top" and "bottom" of the solar panel 102 are not meant as limitations
on where the panel is positioned. The system 101 may comprise a mirror chamber
or similar elements to re-reflect light reflected from the surface 105 or emitted from
the solar panel 102.
[0040] The solar panel 102 may, and generally does comprise, a plurality of

individual solar cells connected in series or parallel. Solar cells are photovoltaic
devices suitable for generating electricity from solar radiation. A solar panel may be
comprise an array of smaller solar panels. The solar panel 102 can be of such a
type, provide each of the smaller panels has a thermoelectric device 103. The
relationship between the solar panel 102 and the thermoelectric device 103, in terms
of their adjacency and heat transfer, would be repeated for each separate element of
the array. A unitary construction of the solar panel 102 and the thermoelectric
device 103 would then be understood to mean unitary with respect to each element
in the array.
[0041] The solar panel 102 (or each unit in the array) is thin, giving it just two
major sides. These may be referred to as front and back or top and bottom. The
front or top surface 105 is the one faced to the light. The top surface 105 and the
bottom surface 106 are substantially conformai, notwithstanding any protrusions.
They have approximately equal gross surface areas.
[0042] The solar panel 102 can comprise any type of photovoltaic cell suitable for
the environment of use. The examples discussed in the background section are
applicable in a broad sense, although adaptations for photovoltaic functioning at high
temperature are preferred. The need for high temperature durability and the ability
to withstand thermal cycling also narrows the practical choices. The solar powered
generator 100 is designed with the intention that solar energy will heat the solar
panel 102 to high temperatures, such as 475 K, 575 K, 675 K, or higher. The
development of high temperatures is encouraged to provide a source of high
temperature heat for the thermoelectric device 103.
[0043] The solar panel 102 is preferably adapted for photovoltaic performance at
high temperatures. Solar panels adapted for operation only under ambient
conditions include bulk single crystal silicon solar panels and commercially available
serially connected multi-junction thin film solar panels. Either type rapidly loses
efficiency with increasing temperatures.
[0044] Adapting the solar panel 102 for high temperature operation generally
comprises selecting high band gap energy semiconductors materials. GaN (3.2 eV),
SiC, GaP (2.26 eV) are examples of semiconductors with high band gap energies
that can be used to form high temperature-adapted solar cells. Temperature
sensitive designs, such as the common serial multi-junction design, are either

avoided or tuned to a very high temperature.
[0045] A solar panel 102 adapted for high temperature performance comprises at
least one semiconductor junction, uppermost in a layered configuration, having a
band gap energy higher than would be selected for operation at ambient
temperatures. Higher band gap energy semiconductors utilize less of the solar
spectrum than lower band gap energy semiconductors, however, higher band gap
energy semiconductors lose less of their efficiency with increasing temperatures than
lower band gap energy semiconductors. High band gap energy solar cells sacrifice
room temperature performance in order to retain more performance at high
temperatures.
[0046] The optimum band-gap energies referred to in the background section are
not those most preferable for the present application. The ideal band gap energy is
application dependent, but a suitable selection can be made based on theory or
experiment. In a single junction solar panel, a band gap energy greater than 1.6
would indicate adaptation to high temperature use and above 1.8 eV more definitely
so. In a dual junction device an upper layer bandgap energy of 2.0 eV is indicative,
with 2.2 eV being even more so.
[0047] A single-junction GaAs single crystal or thin film solar panel is better suited
to high temperature operation than most solar panels, but is not adapted to high
temperature operation. The GaAs band gap energy (1.4 eV) is high in comparison to
that of silicon, making it less temperature sensitive than silicon. As the terms will be
used in this disclosure, GaAs is adapted to medium temperature operation, but not to
high temperature operation.
[0048] In the present disclosure a high temperature is at least 475 K. Operation
with the temperature of the solar panel 102 peaking in excess of 675 Kelvin is typical
for a solar powered generator 100 in order to facilitate electricity generation through
the thermoelectric device 103. Adaptation to operation at such high temperatures
does not mean that the solar panel 102 will not have diminished performance at 475
Kelvin in comparison to performance at an ambient temperature of 300 Kelvin.
Almost any (or every) solar panel will experienced a diminution in efficiency with
increasing temperature. Adaptation to high temperatures comprises sacrificing
performance at ambient temperatures to improve performance at high temperatures.
[0049] A good indication of high temperature adaptation in a serially connected

multi-junction solar panel is the relative current output of the various junctions as a
function of temperature. The junctions are usually connected in series and current
matched. Current matching comprises adjusting the junction layer thicknesses until
each junction produces nearly the same current. When the currents are not
matched, the result is highly detrimental to performance. Because the degree to
which temperature affects current varies widely among the different junctions used in
multilayer devices, current matching must be done for a particular temperature. The
temperature at which each layer produces the same amount of current under solar
illumination is the temperature at which the solar panel is adapted to operate. These
comments about current matching apply to serially connected multi-junction devices.
The need for current matching can be avoided by parallel connection. Parallel
connection is generally not used in multilayer solar panels because of the complexity
of the structures its implementation requires. A compromise for the present
application would be to use parallel connection but limit the maximum number of
junctions to two.
[0050] The solar panel 102 has a low resistivity to transferring heat through its
thickness in comparison to the thermoelectric device 103. If the solar panel 102 is
overly thick or has an inadequate thermal conductivity for its thickness, a significant
temperature gradient develops between its top surface 105 and its bottom surface
106. Some temperature gradient is necessary for transmitting heat to the
thermoelectric device 103, but in a prefer design this gradient will be very small. A
large gradient would not reduce steady state energy production by the thermoelectric
device 103, but it would result in unnecessarily high temperatures in the solar panel
102.
[0051] Depending on the degree of solar concentration used by the solar
powered generator 100, it may be important to consider thermal resistivity in
designing the solar panel 102. At low solar concentrations, ordinary materials are
likely to be adequate although care should be taken that substrates and backing
materials provided with the solar panel 102 do not introduce excessively with thermal
resistivity.
[0052] A low heat capacity for the solar panel 102 is generally advantageous, but
there are advantage in having a high heat capacity. A high heat capacity reduces
temperature fluctuations and rates of temperature change, which reduces stress on

materials and enhances durability. Beyond the durability concern, which may define
a minimum required heat capacity, the considerations are more complex.
[0053] The advantages of a high heat capacity, such as a metal plate might
provide without introducing excessive thermal resistivity, include temperature stability
and more photoelectric energy production at low temperatures. While high heat
capacity means that both high and low temperatures take longer to go away,
transients associated with going from low to high temperature correspond to the
availability of light whereas transients associated with going from high to low
temperatures are associated with sunlight having been lost. The net result is more
light on average at low temperatures and less light on average at high temperatures.
If the solar panel 102 were the only consideration, high heat capacity would be
beneficial.
[0054] The considerations are reversed for the thermoelectric device 103. The
thermoelectric device 103 provides the most efficiency when the solar panel 102 has
arrived at steady state maximum temperature. If all these components rapidly reach
that condition when the sun comes out, energy production from the thermoelectric
device 103 will be maximized. If the warm-up period is long, much heat will be
transferred through the thermoelectric device 103 at a lower temperature differential,
and thus with a lower thermoelectric conversion efficiency. Likewise heat stored by
the solar panel 102 will be transferred during cool down rather than while the
temperature differential is still at its maximum.
[0055] Given the foregoing considerations, the heat capacity in and around the
solar panel 102 is a matter involving several consideration, which are independent
with other design choices. Adaptations for either lowering or increasing heat
capacity may be warranted according to the application.
[0056] Increased heat capacity can be provided at the interface between the solar
panel 102 and the thermoelectric device 103 or above the surface 105. A
transparent cover over the surface 105 having good thermal contact with the surface
105 would be preferred for the advantage of not increasing thermal resistivity
between the solar panel 102 and the thermoelectric device 103, but that advantage
must weighed against any loss in photoelectric generation due to light absorption by
this covering layer.
[0057] Any structure that does not excessively interfere with heat transport

between the solar panel 102 and the hot junction 107 can be used to provide
additional heat capacity, if such additional heat capacity is desired. Suitable
structures include metal layers. Metals have a favorable combination of high heat
capacity and high thermal conductivity.
[0058] A thermoelectric device as the term is used herein is a device comprising
a hot junction and a cold junction and functional to generate electricity directly from
thermal energy when the hot junction is held to a temperature above that of the cold
junction. The thermoelectric device 103 comprises p and n-doped semiconductor
regions. Charge carrier concentrations within these regions depend on temperature.
When a temperature gradient is applied from the hot junction 107 to the cold junction
108, the temperature gradient traverses these regions and thereby creates charge
carrier gradients. The charge carrier gradients result in a flow of electricity.
[0059] Figure 3 provides an example 203 for the thermoelectric device 103. The
thermoelectric device 203 comprises p-leg 219, which comprises at least one n-
doped semiconductor segment, and n-leg 220, which comprises at least one
segment of a n-doped semiconductor region. The legs 219 and 220 span a gap
between a hot junction 207 and a cold junction 208. Electrical insulation 222 isolates
the legs. Metal interconnects 218 are provided adjacent or within the hot junction
207. Ohmic contacts are connect each of the legs 219 and 220 to the metal of
interconnect 218, whereby the p-leg 219, the metal interconnect 218, and the n-leg
220 provide a p-i-n junction. The p-i-n junction creates electric fields through legs
219 and 220. At the cold junction 208, the legs 219 and 220 have ohmic contacts
with leads 221a and 221 b. These leads are electrically isolated from one another
although they may be part of a single metal interconnect layer, an interconnect layer
being a pattern of metal in a planar matrix of dielectric.
[0060] When the hot junction 207 is maintained at a higher temperature than the
cold junction 208, charge carrier gradients form in the legs causing electrons to flow
down the p-leg 219 and holes (effectively) down the n-leg 220. The potential
difference between the leads varies depending on the temperature differential
between the hot and cold junctions. The voltage is approximately proportional to the
temperature difference. The electric current multiplied by the voltage gives the
available power provided by the thermoelectric device 203.
[0061] The ideal efficiency with which such a device converts thermal energy to

electrical energy is given by well-known formulas that show dependencies on the
temperature difference between the hot and cold junctions, the geometry, and the
properties of the materials making up the p and n-legs. The efficiency, q, is given by:

where o is the electrical resistivity, K is the thermal resistivity, and S is the Seebeck
coefficient. 7 is an average temperature. The formulas present here are simplified
by treating ZT as a constant. A more detailed formulation would need to account for
the temperature dependent variation of ZT through each of the legs 219 and 220,
and the fact that the semiconductor material is not the same for each leg, or
necessarily within each leg. This should not detract from the points made below.
[0062] The first term in equation (1) is the Carnot efficiency. The Carnot
efficiency is a consequence of entropy and cannot be avoided by any type of device
for converting thermal energy to electrical energy. The second term in Equation (1)
shows the separation between a thermoelectric device and an ideal device. The
major dependence of this term is the dependence on ZT, with higher values being
better. Until recently, the best values of ZT were about 1.0 and limited the efficiency
of thermoelectric devices to about 20% of the Carnot efficiency.
[0063] Figure 4 shows the figures of merit of several semiconductor materials
over a range of temperatures. It can be seen from this figure that different semi-
conductor materials are effective over different temperature ranges. This
complicated the selection of semiconductor materials, because the thermoelectric
device 103 is intended to operate with the temperature gradient running through the
legs 219 and 220: the temperature is expected to vary greatly over the lengths of the
legs. High temperatures will occur at the tops legs 219 and 220 and low

temperatures at the bottoms. A material that would provide a good figure of merit
may give poor performance at the bottom and vice versa.
[0064] Figure 3 illustrates the preferred solution and a preferred selection of
materials. The solution is to make each of the legs 219 and 220 from a plurality of
segments, each segment corresponding to a different semiconductor material. The
lower segments 219c and 220c, are selected to have high figures of merit at lower
temperatures, and the upper segments 219a and 220a are selected to have high
figures of merit at high temperatures.
[0065] The figures of merit of many semiconductor materials, including those
shown in Figure 4, can be improved greatly by introducing quantum confinement of
carriers. The main effect of such quantum confinement is a substantial reduction in
thermal conductivity. The figure of merit can be nearly doubled in many cases.
Figure 5 provides an exemplary result for comparison with Figure 4. Quantum
confinement is provided by nano-scale structures formed in the semiconductor
matrix. For the compiste of Figure 5, the nanostructures are particles of p-SiGe
crystal in of mixed sizes in the range from 1-200 nm. Nano-scale structure include
quantum wells, quantum wires, and quantum dots. Quantum dots provide the most
benefit. These nano-scale structures are composite structures: the dots are small
crystals of a second, suitably selected, semiconductor material formed in the matrix
of another semiconductor. Suitable materials for the nanostructure can be found for
each of the matrix materials. Some additional examples include Bio.3Sb1.7Te3 for a
[0066] One technique for forming the quantum dots is to alternate between
depositing a few layers of the matrix materials and depositing a few layers
comprising the matrix material together with wells of the second material. Details of
suitable techniques are in the public domain. The materials and methods that have
been described by workers at the Massachusetts Institute of Technology (MIT) are
recommended. Those not familiar with these materials and methods can find them
described in various publications including US patent number 6,444,896 and U.S.
Pat. Pub. Nos. 2006/0118158, 2008/0202575, 2009/0068465. Those patents and
patent publications are incorporated herein by reference in their entirety.
[0067] Parameters that can be tuned in order to improve the result include
thickness of the quantum well structures, their interspacing, and the atomic
proportions of the alloys. Segmentation as shown in Figure 3 remains desirable for

the nanocomposites, although it will be noted that the composites have their peak ZT
values in temperature ranges shifted from those for the source materials. Adjusting
the composition of the nanostructure alloy is one way to shift the temperatures at
which high ZT values occur.
[0068] Two parameters are available for balancing the heat flux and energy
production of the legs 219 and 220. One type of leg can be made wider than the
other; width being meant in the sense of greater cross-sectional area. The other
mode of adjustment is to place a pedestal under one or the other leg so that one leg
can be shorter than the other. A pedestal is a leg segment made of a thermally
conductive material such as a metal.
[0069] The structure of the solar powered generator 100 determines the
magnitude of the temperature gradients that will develop across the thermoelectric
device 103. The main factors that determine that gradient are the solar
concentration factor, f, and the thermal resistivity of the thermoelectric generator
103. That thermal resistivity can be tuned by adjusting the height of the legs 219
and 220.
[0070] The solar concentration f, together with the intensity of the sun's radiation
determine the required heat flux. The entire sun's spectrum to the extent practical
will be concentrated and focused on the surface of the solar panel 102. While some
of the energy will be converted to electricity, the great majority, typically 90% to 95%,
will become thermal energy. The configuration is for all the heat to go in one
direction, downward, perpendicular to the surfaces of the solar panel 102. The
thermoelectric device 103 is conformal with the solar panel 102. The energy flux
density through the thermoelectric device 103 is nearly the same as that through the
bottom surface of the solar panel 102.
[0071] The maximum in the heat flux rate through the thermoelectric device 103
is given by the peak intensity of the sun's radiation on the Earth's surface, about 1.3
kW/m, multiplied by solar concentration factor, f. Corrections can be made for
energy conversion to electricity by the solar panel 102 and parasitic (unintended)
heat losses, but the result is still roughly the amount of heat flux per unit area that
the thermoelectric device 103 must be designed to transport.
[0072] An important design choice to consider at this point is the target for the
temperature differential, ΔT, between the hot junction 207 and the cold junction 208.

Larger temperature differential lead to more efficient thermoelectric energy
production, lower temperature differentials lead to a cooler solar panel 102, and
more photovoltaic energy production.
[0073] The preference here is to choose a large ΔTto get into the range where
thermoelectric conversion efficiency is high, and the sensitivity of the total energy
production to the cold junction temperature is low. Preferably, ΔT is at least 200 °C,
more preferably at least 300 °C. Higher values such as 500 °C and 600 °C can be
desirable in that ΔT remains high even when the light level has dropped well below
its peak. The main disadvantage of going to higher and higher temperature
differentials is that peak temperatures are increased and materials begin to
deteriorate and eventually fail.
[0074] The approximate leg height, h, to achieve a target temperature differential,
AT, can be calculated as follows:

where K is a suitably calculated average thermal resistivity for the legs 219 and 220.
From this formula, it can be seen that high thermal resistivity allows the legs to be
shorter. A high solar concentration factor f greatly reduces the required height of the
legs. Materials costs can be a significant contribution to the total cost of this system,
and reducing the required amount of semiconductor material is very advantageous.
Equation (4) shows that a solar concentration factor of 100 reduces the required
thickness for the semiconductor material in the thermoelectric device 103 by a factor
of 100. This is compounded with the gain of processing 100 times as much sunlight
per unit area. The total reduction in the thermoelectric material requirement is
approximately f squared, 10,000 in this example. Solar concentration makes feasible
the use of materials that would otherwise be too expensive. For this reason, low
solar concentration is preferred over no solar concentration, moderate solar
concentration is more preferred, and high solar concentration is still more preferred.
Another advantage of solar concentration is that it facilitates reaching the target
temperatures differential quickly; it has an effect similar to making the heat capacities
less.
[0075] Preferably, all the heat going from the hot junction 207 to the cold junction

208 travels through the legs 219 and 220. Any heat traveling through the insulation
222 does not contribute to thermoelectric energy generation. The materials of legs
219 and 220 are themselves generally good insulators, even if they are just ordinary
semiconductors. When they are made into nano-composites they become even
better insulators: nano-structures improve the thermoelectric figure of merit by
increasing the thermal resistivity more than the thermal conductivity of the composite
material. There are only a few types of materials that are substantially better
insulators. In particular, substantially better insulators are air, vacuum, and aerogels.
Here, "vacuum" is encompassed within the term "insulating materials".
[0076] The thermoelectric device 103 is conformal with the solar panel 102 in
order that it drains heat uniformly from the back 106. This dictates the cross-
sectional area for the device 103. The thermal resistivity offered by the
thermoelectric device 103 is also constrained. If the thermal resistivity is too low, the
desired temperature difference will not develop. If the thermal resistivity is too high,
the solar panel 102 will heat excessively.
[0077] A concept for reducing the amount of semiconductor material required for
the thermoelectric device 10 and indeed for any thermoelectric device that is
designed to provide a predetermined heat flux per unit area with a predetermined
temperature gradient, is illustrated by Figure 7, which corresponds to a cross-section
of the thermoelectric device 203 through the plane of line A-A'of Figure 2.
[0078] According to this concept, a majority of the cross-sectional area (at least
50%), and volume, preferably at least 90%, is filled with a highly effective insulating
material, preferably a material selected from the group consisting of vacuum, air,
aerogel. This reduces the area available for heat conduction through the legs 219
and 220 to a minority of the area (less than 50%), preferably less than 10%. If the
cross-sectional area of the legs 219 and 220 is reduced by 50%, the thermal
resistivity between the hot junction 207 and cold junction 208 is approximately
doubled. To maintain the design temperature differential and heat flux the heights of
the legs, h, are halved. The adjustment is applied to each segment if multiple
segments are used. Halving the cross-sectional area and halving the height, h,
reduces the amount of material required by 75%. Reducing the cross-sectional area
by 90% and reducing the height by 90% reduces the amount of required
thermoelectric semiconductor material by a factor of 100. This is a particularly

important advantage if expensive materials are used.
[0079] In order to prevent excessive temperature gradients from developing
around the hot junction 207, the legs 219 and 220 are evenly and closely spaced
through the matrix of insulation 222 throughout the entire area of the thermo-electric
device 203. Close spacing can be maintained while reducing the fraction of the area
occupied by the legs 219 and 220 by increasing the number of legs as their sizes are
reduced. It should be appreciated that there are innumerable ways to meet these
geometric constraints. For example, the legs 219 and 220 can have cross-sections
that are elongated like wires.
[0080] As the percentage of area occupied by the insulation 222 becomes
greater, heat loss through the insulation becomes more significant and will eventually
outweigh the benefit of further reduction in semiconductor area and material usage.
Limitations on material processing can also pose a limit, as well as changing
electrical properties with dimensions, including the properties that change
unintentionally as attempts are made to shape progressively smaller structures.
Nevertheless, the structure illustrated by Figure 7 is an enabler to the use of nano-
well composites that are made by process comprising separate steps for depositing
each of many nanometers thick layers. Being able to use shorter legs 219 and 220
reduces the required number of process steps.
[0081] A heat sink 104 can be anything that is functional to continuously draw
heat from the cold junction 108 at such a rate that the solar powered generator 100
can arrive at steady state operation under continuous full sun. The heat sink 104
can comprise a fixed body of material, a body of water for example, or comprise a
heat exchanger that transfers heat from the cold junction 108 to an essentially
inexhaustible flow, as in the case of a heat exchanger with fins and a steady stream
of air. While the solar generator 100 is not limited with respect to the type of heat
sink 104, it has particular utility for a certain class of heat sink.
[0082] One such class of heat sink is a partially closed system of such limited
capacity to take up heat that under continuous full sun the heat sink 104 is warmed
significantly by the solar powered generator 100. Significance in this context means
a substantial change in the consequent temperatures within the solar generator 100.
For example, a significant change would cause the cold junction 108 to become at
least 40°C hotter, and a more significant change wouid cause the cold junction 108

to become at least 100 °C hotter. Such changes will affect steady state
temperatures throughout the solar powered generator 100. Any change in the
temperature of the heat sink 104 that causes the temperature of the solar panel 102
to rise by 40 °C or more is significant because of the reduction in efficiency that this
would cause if ordinary solar panel materials were used. In this regard a change of
100 °C would be very significant.
[0083] When the capacity of the heat sink 104 is so limited that temperature
changes of these magnitudes are routine, the solar powered generator 100 can
provide a valuable improvement in comparison to a conventional solar thermal
cogeneration system, which would lack the thermoelectric device 103 or a design
that encourages heating more than a few degrees above the desired hot water
temperature. The solar powered generator 100 is designed for the thermoelectric
device 103 to be the predominant mode of electric power generation. The generator
100 remains highly functional in the face of temperature changes that would
undermine the efficiency of a conventional solar panel.
[0084] Accordingly, according to a method of the invention, the solar power
generator 100.is connected to a heat sink 104 that is by its nature limited in
effectiveness or variable in its temperature, whereby the solar powered generator
100 will operate with the temperature of the cold junction 108 varying by 40 Kelvin or
more, optionally 100 Kelvin or more. The method comprises developing a large
temperature gradient across the thermoelectric device 103, whereby the
thermoelectric device 103 is the predominant mode of electricity generation,
producing more power than the solar panel 102. The large temperature gradient is
brought about by exposing the solar panel 102 to sunlight light with a sufficiently high
solar concentration"factor, f. What constitutes a sufficiently high solar concentration
factor depends on the thermal resistivity of the thermoelectric device 103, which is
selected to allow the desired gradient to be generated with the solar concentration
factor f that is provided by or achievable with the solar concentration system 101.
The advantage of the method is that it provides power generation with an efficiency
that has a low sensitivity to fluctuations in the heat sink performance and the cold
junction temperature.
[0085] A heat sink 104 having limited capacity could be a domestic hot water
system, depending on the amount of water it contains in relation to the capacity of

the generator 100. If the solar powered generator is merely a supplemental heating
system whereby the hot water remains at an essentially constant temperature, the
heat sink 104 will be efficient and an ordinary solar thermal generator would most
likely better serve. On the other hand, if the hot water temperature varies between
50°C and 95°C, or 25 °C and 95°C, in the necessary course of its functioning as the
heat sink, than it is a heat sink of limited capacity.
[0086] Other types of heat sinks that may motivate the use of the solar generator
100 include those that vary widely in temperature of their own accord, either in terms
of heat sink temperature or heat transfer coefficient. For example the heat sink 104
could be a cooling system of a vehicle. When the vehicle's engine is stopped the
solar generator 100 can serve to keep the coolant, providing easy starts on cold
days and reducing cold start emissions. Another potential use would be cabin
heating while the engine is not running, and thereby avoiding the need to idle the
engine. If enough cooling can be provided to prevent overheating, by running the
engine fan of example, an air conditioner can be powered with the engine off.
[0087] When the vehicle is running, the solar generator 100 can provide auxiliary
power and thus improve the vehicle's efficiency. If there is a danger of overheating,
running an engine fan may be enough. These uses make do with a heat sink that is
highly variable and can exceed 100 °C in temperature. The solar powered generator
100 will typically operate efficiently with a heat sink in the temperature range from
100°C to 200°C.
[0088] To prevent overheating, means can be provided to slow or shut down the
solar powered generator 100. A solar tracker would be a suitable means, if one is
used. A tracking system can orient the collector away from the sun if that becomes
desirable to prevent overheating. If the solar concentration system 101 provides a
controllable solar concentration factor, f, that factor can be reduced. In general,
however, it is preferred that the solar concentration system 101 provide as much
light to the solar panel 102 as it is capable of providing.
[0089] Another application that can involve a vehicle uses airflow for cooling. In
this example, the heat sink 104 is a heat exchanger that transfers heat from the cold
junction 108 to the ambient: Such a heat sink may have a highly variable
performance that depends on ambient temperature and whether the vehicle is
moving or stopped.

[0090] Depending on the relative sizes of the solar generator 100 and the heat
sink 104, the heat sink 104 may be inadequate to maintain a sufficiently constant
temperature to maintain a conventional solar panel in an efficient mode of operation.
The solar powered generator 100 can make do with much less cooling as compared
to a similar device using just a solar panel. The solar powered generator 100 can be
designed to operate efficiently over a broad range of heat sink temperatures.
Designing for a high temperature gradient through the thermoelectric device 103
shifts power generation to the thermoelectric device 103 and reduces reliance on the
solar panel 102.
[0091] The effective functioning of the solar panel 102 can be limited to warm-up
periods during which the thermoelectric device 103 has not developed the
temperature gradient it requires to operate efficiently. The solar powered generator
100 could be used to drive a vehicle or as part of a hybrid drive system. In such a
system, it is advantageous to have electric power as soon as the sun comes out.
[0092] The thermoelectric generator 103 produces its current at a voltage that
varies with the temperature difference between hot junction 107 and the cold junction
108. The solar powered generator 100 receives varying amounts of light over the
course of any given day, whereby the heat flux, temperature gradient and resulting
voltage will necessarily vary substantially. Therefore, it is preferred that the solar
powered generator 100 be provided with an electrical system including a voltage
regulator for drawing the current from the thermoelectric device 103 at the coltage it
is supplied and outputting that current at a constant voltage.
[0093] The solar panel 102 provides power separately from the thermoelectric
generator 103. It will generally be desired to combine the output from the solar panel
102 with that of the solar generator 103 in order to form a single source. This is
accomplished by providing the solar powered generator 100 with electrical
components for combining and matching the voltages from these two sources.
[0094] Optionally the solar powered generator 100 includes an electrical energy
storage system. This energy storage system may comprise batteries and or
capacitors. Another option that may be useful is a standard coupling for plugging in
to the solar powered generator 100. A transformer may also be included to convert
the direct current into an alternating current with a standard frequency and voltage.
[0095] When the solar powered generator 100 is operating, the solar

concentration system 101 concentrates sunlight 109 onto the surface 105 of the
solar panel 102. The solar panel 102 absorbs most of this sunlight (radiation) and
from it produces electrical energy with an efficiency that diminishes with increasing
temperature. Most of the absorbed radiation is converted to thermal energy.
[0096] The bottom surface .106 of the solar panel 102 abuts the thermoelectric
device 103. The thermoelectric device 103 comprises hot junction 107 and cold
junction 108. The hot junction 107is proximate to and in close thermal contact with
the bottom surface 106 of the solar panel 102. In this arrangement the
thermoelectric device 103 provides the primary pathway for which the solar panel
102 gives up its heat. If necessary, the solar panel 102 can be contained within an
enclosed and/or insulated space to reduce other pathways of heat loss. By
eliminating or reducing other pathways of heat dissipation, the vast majority of the
thermal energy absorbed by the solar panel 102 can be directed through the
thermoelectric device 103, whereby it can be used for electrical power production.
[0097] Some of the incident radiation will be reflected from the solar panel 102.
In addition, the solar panel 102 will release energy through radiation. Reflectors may
be positioned to redirect this emitted and reflected light to re-reflect the light back
upon the surface 105. These reflectors may comprise a mirror chamber substantially
a space over the surface 105. Such reflectors can provide a significant increase in
efficiency.
[0098] Heat from the hot junction 107 flows to the cold junction 108. A portion of
the thermal energy transported in this way is converted to electricity by the
thermoelectric device 103. Thus the solar powered generator 100 generates
electricity in at least two places. The electrical power from these sources can be
transformed and combined in order to provide a unitary power supply at a constant
voltage using electronic parts separate from or integrated into the solar powered
generator 100.
[0099] Heat sink 104 draws heat away from the cold junction 108. The heat sink
104 may be highly efficient and maintain the cold junction 108 at an essentially
constant temperature regardless of the intensity of the sunlight 109. Alternatively,
the heat sink 104 may be inefficient whereby the temperature of the cold junction
108 varies. The temperature of the hot junction 107 varies correspondingly, as the
temperature differential is determined by the heat flux rate, which is substantially

independent of the cold junction temperature. When the cold junction temperature
rises, the hot junction temperature rise to match. The hot junction temperature rises
until the heat input to the solar cell 102 matches the heat output to the hot junction
108. This matching occurs at approximately the same temperature differential
regardless of the cold junction temperature. Thus, an increase in the cold junction
temperature soon leads to approximately equal increases in the temperatures of the
hot junction 107 and the solar panel 102.
[0100] The performance of solar powered generator 100 is illustrated by the finite
state machine diagram 239 of Figure 6. The generator 100 begins in the inactive
state 240. In the inactive state 240, all of the components of the solar powered
generator 100 are near ambient temperature. In this regard, it should be appreciated
that the main focus of the present application is devices for use in terrestrial
applications. The inactive state 240 is typical for nighttime.
[0101] The main event that causes a departure from the inactive state 240 is the
sun rising. This event moves the device 100 to the low-temperature operating state
241. In the low-temperature operating state 241, the solar panel 102 is producing
power near its peak efficiency, while the thermoelectric device 103 provides little or
no power.
[0102] While light levels remain low, the generator 100 remains in the low-
temperature operating state 241. When light levels increase, the generator 100
immediately begins to produce more power. The increased light levels quickly warm
the solar panel 102 and the generator 100 transitions to the middle temperature
operating state 242. Power production by the thermoelectric device 103 will
generally surpass power production by the solar cell 102 as the generator 100
warms to state 242.
[0103] As the solar panel 102 warms, the temperature gradient across the
thermoelectric device 103 increases. The efficiency of the solar panel 102
decreases while power production by the thermoelectric device 103 increases.
Preferably, in the middle temperature operating state 242, as temperatures fluctuate,
efficiency losses to the solar panel 102 are balanced by efficiency gains by the
thermoelectric device 103, and visa-a-versa, whereby efficiency remains in a narrow
range even as light and power levels fluctuate. Optionally, however, the efficiency of
the solar panel 102 drops during this period to a range in which power production by

the solar panel 102 is very low in comparison to power production by the
thermoelectric device 103, and the overall efficiency is substantially that of the
thermoelectric device 103, which by and large increases monotonically with
temperature.
[0104] With persistent full sunshine, the solar powered generator 100 reaches the
high temperature steady-state 243. The temperature of the solar panel 102 is about
its designed maximum, although the temperature of the solar panel 102 also
depends on that of the heat sink 104, which can vary. The temperature gradient
through the thermoelectric device 103 is also at approximately the designed
maximum, depending on such factors as the season and time of day. Light input is
near its maximum and efficiency of the thermoelectric device 103 is near its peak.
The efficiency of the solar panel 102 is diminished to a degree that is either
moderate or very great, depending on whether the solar panel 102 is adapted for
high temperature operation. Even with the solar panel 102 adapted for high
temperature operation, it would is typical for the thermoelectric device 103 to
produce 2, 3, or 4 times as much power as the solar panel 102.
[0105] Efficiency as well as power production in the solar powered generator 100
are typically highest in the high temperature steady state 243. Materials
considerations are likely to be the factor militating against designing for even higher
temperatures and temperature gradients and thereby accessing higher levels of
efficiency.
[0106] A rapid transition to the high temperature steady state 243 when full sun
becomes available will generally provide greater efficiency. This transition is slowed
according to the thermal mass of the solar panel 102 and any materials used to
make contact between the solar panel 102 and the thermoelectric device 103. Where
the panel 102 and the thermoelectric device 103 are manufactured separately,
solder or thermal paste may be used to ensure a good contact. A small air gap at
the interface could cause the temperature of the solar panel 102 to rise significantly
above that of the hot junction 107, which would reduce the performance of the solar
panel 102 without providing any benefit to the thermoelectric device 103. A more
severe breakdown in contact could quickly result in a damaging temperature
excursion, particularly if solar concentration factors. Such a breakdown could be
caused by a deformation of one of the contact surfaces, which might itself be the

result of temperature cycling.
[0107] A concept that solves several of these problems is provided by a solar
panel 202 and a thermoelectric device 203 having a unitary construction. In such a
construction, the solar panel 202 and the thermoelectric device 203 are layers in a
composite structure similar to an integrated circuit. The layers are formed one over
the other through a sequence of process steps. Processes of masking, etching, and
deposition may be combined in various ways to produce the desired result. Most of
the steps are, or can be, the same as those normally used to form the devices 102
and 103 individually, particularly with regard to the case where the solar panel 102 is
a thin film solar cell and the thermoelectric device 103 comprises nano-composite
segments formed through many separate layer deposition steps.
[0108] The process of making the integrated device 200 may begin from the
bottom up or the top down. Starting from a point where the thermoelectric device
203 is formed, the principle modification is to ensure the interconnect layer 207 is
planar and to deposit a semiconductor substrate layer over the interconnect layer in
such a way that the substrate does not easily delaminate. One approach is to
deposit a metal layer over the interconnect layer 207. Forming a GaP solar cell over
such a metal layer is a conventional process.
[0109] Alternatively, the solar cell can be formed first. One option is to use a
temporary substrate, Ge for example, as a structure on which to build the solar cell.
All or just the bottom layers of the solar cell 202 are formed on the Ge substrate.
Then the layers comprising the thermoelectric device 203 are built. Finally, the
temporary substrate is removed, and if necessary, the solar cell 202 given additional
processing to become completed.
[0110] The integral construction provides a variety of advantages, including
higher thermoelectric efficiency due to less thermal mass, excellent thermal contact
between the solar panel 203 and the hot junction 207, and a better ability to with
stand thermal cycling through a reduction in the number of layers, and the avoidance
of thick layers.
[0111] An example application for the solar powered generator is the system 900,
which is a combined system for domestic solar hot water heating with cogeneration
of electricity. The system 970 includes the solar collector 971, which is designed for
rooftop installation. The collector 971 gathers solar energy 109 and transmits it via

the fiber optic cable 972 to the solar powered generator 900. The size of the
collector 971 is chosen in relation to the amount energy that an ordinary household
will use for hot water heating. A size in the range from about 1 to 10 square meters
could be suitable, typically in the range from about 2 to about 6 square meter.
[0112] The solar powered generator 900 is located inside a building 973,
preferably next to the water tank 974. Water tanks are usually in a basement 975
and it is preferable to avoid the energy waste of circulating hot water through long
pipes. An location in attic 976 may be preferable for ease of installation. A suitable
size for the tank 974 would be in the range from 100 to 1000 liters, more typically in
the range from about 200 to about 600 liters.
[0113] The fiber optic bundle 972 shines light onto the surface of the solar panel
of the solar powered generator 900, preferably with a high degree of solar
concentration. An intensity in the range from 50 to 250 suns would be preferred. At
100 suns, the solar panel would be in the range from 100 to 1000 cm2. A mirror
chamber comprising mirrors on the inside and surrounded by vacuum insulated
double-pane glass, traps heat and light rising off the solar panel. At steady state
operation with full sun, the solar panel develops its maximum designed temperature
gradient, which is 350 Kelvin.
[0114] The heat from the solar panel is transmitted by a thermal-electric
generator to the heat exchanger 978. The heat exchanger 978 forms part of a loop
979 through which water re-circulates between the tank 974 and the heat exchanger
978. By placing the solar powered generator 900 near the base of the tank the re-
circulating flow can be powered thermo-gravimetrically. Alternatively, an electric
pump could be used.
[0115] The hot water system comprising the heat exchanger 978, the loop 979
and the tank 974 provide the heat sink that is a closed system with a limited capacity
to absorb heat. Fluctuations in user demand are accommodated in part by allowing
the water temperature to vary through a wide range. A back-up gas heater may
bottom the generator 900 to provide a minimum temperature of 50 °C. The water can
then be allowed to heat to 95 °C before taking any measures to release the excess
heat. A mixing valve 980 is configured to automatically adjust a mixing ratio between
water from the tank 974 and a cold water supply 981 to provide water on demand at
a preset temperature. If the hot water temperature reaches its allowed maximum,

heat can be surrendered by dumping hot water or defocusing the solar collector 971.
As the heat sink cycles from 50 °C to 95 °C, the solar panel is expected to cycle from
400 °C to 445 °C. This cycling is not expected to affect electrical power production.
[0116] Using a single junction thin film GaP solar cell, and conventional
thermoelectric materials, power generation from the solar panelat room temperature
would be about 10%. As the device 900 increases to its operating temperature, the
solar panel production efficiency diminishes to about 7%. Generation from the
thermoelectric device increases with temperature. If conventional thermoelectric
semiconductors with 0.8 ZT are employed, the thermoelectric device efficiency is
expected to reach about 9% at steady state, giving a total efficiency of 16%. If nano-
composite materials are used to provide 2.0 ZT, the efficiency of the thermoelectric
device alone is 16% and the total efficiency is 25%. If the solar panel temperature
drops 100 °C, the overall efficiency only drops to 23%. Power would drop more
because the temperature drop is caused by diminished light. The efficiency drops off
more rapidly at lower temperatures, but remains high over a substantial range of light
levels and water tank temperatures. The high temperature differential design, The
adaptation of the solar panel to high temperature performance, and the efficient use
of semiconductors that enable the use of high ZT materials combine to provide a
system that is economical and efficient.
Industrial Applicability
[0117] The present invention is useful for green energy production.

WE CLAIM
1. A solar powered generator (100) (100), comprising:
a solar panel (102) having a top side (105) and a bottom side (106) and
being functional to generate electric power from sunlight (109);
a thermoelectric device (103) adjacent to and below the solar panel
(102), the device (103) comprising a hot junction (107), a cold junction (108), the hot
junction (107) in close thermal coupling with the bottom side (106) of the solar panel
(102), the thermoelectric device (103) being functional to generate electric power
when the hot junction (107) is hotter than the cold junction (108); and
one or more electrical connections for electrically coupling to and
drawing electrical power from the solar panel (102) and for electrically coupling to
and drawing electrical power from the thermoelectric device (103);
wherein the thermoelectric device (103) comprises n-type legs (219)
configured to conduct heat from the hot junction (107) to the cold junction (108) and
p-type legs (220) configured to conduct heat from the hot junction (107) to the cold
junction (108);
the n-type legs (219) comprise one or more segments of n-doped semi-
conductor materials;
the p-type legs (220) comprise one or more segments of p-doped semi-
conductor materials; and
at least one of the segments is formed of a nano-composite material in
which quantum confinement of carriers substantially reduces the segment's thermal
conductivity.
2. The solar powered generator (100) of claim 1, further comprising a
solar concentration system (101) configured to concentrate sunlight (109) onto the
top of the solar panel (102), the system (101) having a maximum solar concentration
factor, f.
3. The solar powered generator (100) of claim 2, wherein f is 100 or more.
4. The solar powered generator (100) of claim 3, wherein the a solar

panel (102) comprises thin film solar cells of the single or dual junction thin film type,
the cells having an upper most junction of a semi-conductor material that has a
band-gap energy greater than 1.8 eV in the single junction case or 2.0 eV in dual
junction case.
5. The solar powered generator (100) of claim 2, wherein the power
generator (100) is configured in terms of f and the thermal resistivity of the
thermoelectric device (103) for the solar panel (102) to heat to temperatures in
excess of 675 K under the influence of sunlight when operating under ambient
conditions at the Earth's surface, with the cold junction (108) temperature maintained
below 100 °C by a heat sink (104).
6. The solar powered generator (100) of claim 2, further comprising:
a heat sink (104) in contact with the cold junction (108);
wherein the power generator (100), including without limitation f, the
thickness and thermal conductivity of the thermoelectric device (103), the size and
type of the heat sink (104), and the materials of construction, are configured,
adapted, and functional by design for continuous operation of the power generator
(100) under ambient conditions and sunny skies at the Earth's surface to raise the
t solar panel (102) to a temperature at which the thermoelectric device (103) produces
at least twice as much power as the solar panel (102).
7. The solar powered generator (100) of claim 2, further comprising:
a heat sink (104) in contact with the cold junction (108);
wherein the power generator (100), including without limitation f, the
thickness and thermal conductivity of the thermoelectric device (103), the size and
type of the heat sink (104), and the materials of construction, are configured,
adapted, and functional by design for continuous operation of the power generator
(100) under ambient conditions and sunny skies at the Earth's surface to raise the
solar panel (102) to a temperature of at least 575 K.
8. The solar powered generator (100) of claim 1, wherein at least one of
the n-type (219) and p-type legs (220) comprises two segments (219a, 219b, 219c,

220a, 220c) of differing composition, each composition,having an operating
temperature range in which its thermoelectric figure of merit is superior to that of the
other.
9. The solar powered generator (100) of claim 1, further comprising:
a heat sink (104) in contact with the cold junction (108);
wherein the heat sink (104) comprising hot water pipes (979), a hot
water tank (974), and a closed loop (974, 979, 978) through which water re-
circulates between the hot water tank (974) and the cold junction (108).
10. The solar powered generator (100) of claim 9, further comprising:
a solar collector (971); and
one or more optical fibers (972) configured to transmit solar energy
from the solar collector (971) to the solar panel (102);
wherein the solar panel (102), the thermoelectric device (103), and the
hot water tank (974) are located within a residential or industrial building (973)
configured to utilize hot water from the tank (974).
11. The solar powered generator (100) of claim 10, further comprising:
a water mixing system (980) coupled to the hot water tank (974) and a
cold water supply (981);
wherein the mixing system (980) is configured to draw water separately
from the hot water tank (974) and the cold water supply (981) and to supply a
mixture of water (982) drawn from those sources (974, 981); and
the mixing system (980) is configured to automatically adjust a mixing
ratio between water from the tank (974) and water from the cold water supply (981)
as necessary to keep the water supplied by the mixing system (982) below a preset
maximum temperature.
12. The solar powered generator (100) of claim 2, wherein
wherein the concentration factor, f, is 10 or more, whereby the solar
concentration system (101) provides greater than 10 kilowatts per square meter of
solar radiation to the solar panel (102) under clear skies with the sun overhead;

wherein the hot junction (107) of the thermoelectric device (103) is in
close thermal coupling with the back side (106) of the solar panel (102), with the
system being configured for heat transport through the thermoelectric device (103) to
be the principal pathway for cooling the solar panel (102) during steady state
operation;
wherein the thermoelectric device (103) has a heat transfer coefficient
between its hot and cold junction (108) less than (f/100) kilowatts per m2 (solar panel
(102) area) per Kelvin at room temperature, whereby the system is configured to
operate with the solar panel (102) at steady state temperatures 100 Kelvin or more
above the cold junction (108) temperature on sunny days.
,13. The solar powered generator (100) of claim 12, wherein f is 100 of
more.
14. The solar powered generator (100, 200) of claim 1, wherein either the
generator (200) comprises layers of semiconductor material grown on components
of the thermoelectric device (203) or the thermoelectric device (203) comprises
layers of semiconductor material grown on components of the photovoltaic device
(202).
15. The solar powered generator (100, 200) of claim 1, wherein:
the thermoelectric device (203) has a cross-section (A-A') parallel to
the back (106) of the solar panel (202) with an area approximately equal to that of
the back of the solar panel (202), the cross-section cutting through the legs (219,
220) of the thermoelectric device (203);
the legs (219,220) occupy less than 10% of the area of the cross-
section; and
greater than 90% of the cross-section having a highly thermally
insulating composition (222) selected from the group consisting of vacuum, gas, and
an aerogel, the insulating composition filling space between the legs (219, 220).
16. A solar powered generator (100, 200), comprising:
a solar panel (202) having a top side (105) and a bottom side (106) and

being functional to generate electric power from sunlight (109);
a thermoelectric device (203) adjacent to and below the solar panel
(202), the device comprising a hot junction (127), a cold junction (208), the hot
junction (207) in close thermal coupling with the bottom side (106) of the solar panel
(202), the thermoelectric device (203) being functional to generate electric power
when the hot junction (207) is hotter than the cold junction (208); and
one or more electrical connections for electrically coupling to and
drawing electrical power from the solar panel (202) and for electrically coupling to
and drawing electrical power from the thermoelectric device (203);
wherein the thermoelectric device (203) comprises n-type legs (219)
configured to conduct heat from the hot junction (207) to the cold junction (208) and
p-type legs (220) configured to conduct heat from the hot junction (207) to the cold
junction (208);
the n-type legs (219) comprise one or more segments of n-doped semi-
conductor materials;
the p-type legs (220) comprise one or more segments of p-doped semi-
conductor materials; and
either the solar panel (202) comprises layers of semiconductor
material grown on components of the thermoelectric device (203) or the
thermoelectric device (203) comprises layers of semiconductor material grown on
components of the photovoltaic device.
17. A solar powered generator (100, 200), comprising:
a solar panel (202) having a top side (105) and a bottom side (106) and
being functional to generate electric power from sunlight (109);
a thermoelectric device (203) adjacent to and below the solar panel
(202), the device (203) comprising a hot junction (207), a cold junction (208), the hot
junction (207) in close thermal coupling with the bottom side (206) of the solar panel
(202), the thermoelectric device (203) being functional to generate electric power
when the hot junction (207) is hotter than the cold junction (208); and
one or more electrical connections for electrically coupling to and
drawing electrical power from the solar panel (202) and for electrically coupling to
and drawing electrical power from the thermoelectric device (203);

wherein the thermoelectric device (203) comprises n-type legs (219)
configured to conduct heat from the hot junction (207) to the cold junction (208) and
p-type legs (220) configured to conduct heat from the hot junction (207) to the cold
junction (208);
the n-type legs (219) comprise one or more segments of n-doped semi-
conductor materials;
the p-type legs (220) comprise one or more segments of p-doped semi-
conductor materials;
the thermoelectric device (203) has a cross-section (A-A') parallel to
the back (106) of the solar panel (202) with an area approximately equal to that of
the back (106) of the solar panel (202), the cross-section cutting through the legs of
the thermoelectric device (203);
the legs (219, 220) occupy less than 10% of the area of the cross-
section; and
greater than 90% of the cross-section having a highly thermally
insulating composition (222) selected from the group consisting of vacuum, gas, and
an aerogel, the insulating composition filling space between the legs (219, 220).

A solar powered generator (100) has thermoelectric elements adjacent to and
below solar cells. Concentrated sunlight is provided. A heat sink (104),
which can be variable in temperature or efficiency, is in contact with the
cold junction (108) of the thermoelectric device (103). The thermal
resistivity is designed in relation to the energy flux, whereby the
thermoelectric device (103) develops a gradient of several hundred Kelvin.
Preferably the solar cell comprises a high band gap energy semi-conductor.
The generator (100) maintains relatively consistent efficiency over a range
of cold junction (108) temperatures. The heat sink (104) can be a hot water
system. High efficiencies are achieved using nanocomposite thermoelectric
materials. Evenly but thinly dispersing the thermoelectric segments in a
matrix of highly insulating material reduces the amount of material required
for the segments without sacrificing performance. A unitary construction of
the solar cell and thermoelectric elements provides further advantages.